Frequency control system



May 26, 1953' Filed Dec. 17. 1948 2 Sheets-Sheet 1 Frequency '6 l0Controlled Oscillator 5 o 2f 814 ,u ,12 ,13 44 Standard Frequencyv TunedZero Beat 3f f Frequency Mumph" A.F.C.

Source 1 2 2 Circuit f 2f- %L ii '2 2 2 X Faclor=3 i I Frequency Flg 2.Controlled Oscillator )f 3f. ....'.J A llu J20 1 l3a L4a Standard'Frequency Tuned Zero Beat Frequency Multiplier Source 2f 3f'2f=f f--Circuit X Factor=2 Frequency |6 tgmlrlolled 5 sci alor -;,.Q

' v ,llb flab ,l3b ,l4b

Standard Frequency Tuned Mixer Frequency r Multiplier Source A.F.G. nf-fnf+f Circuil 2 i T nfl 2 2 X Factor nf- -f WITNESSES: INVENTOR Sheldoni. Rambo.

' AT RNEY May 26, 1953 S. I. RAMBO FREQUENCY CONTROL SYSTEM Filed Dec.17, 1948 '2 Sheets-Sheet 2 Frequency 0C ('6 H 4 Controlled j r g.Oscillator l5 (ml)? "(n-0f l "C [A26 1 z|3 f Frequency Tuned Mixer ZeroBeat gq y Multiplier (n+l)f-nf =f A.F.C.

ource f f nf (n |)f=f C|rcu|t X Factor= n I lsd X Fig.5. l

Frequency Frequency "lodd l5dd Controlled Controlled OscillatorOscillator Standard Frequency" Tuned Mixer Frequency Multiplier f m -nfA.F.C. Source T circuit mf-nf 2 X Factor F 6 l5e g&

q we Frequency i- Frequency tofltlilifll'ad gontnalled sci a or sci a orf J F 1k. ,lle 7 l3: 7 l4e smndm'd Tuned Mixer Zero Beat Frequency AECSfOllCG 5 Circuit ,llee- Standard Frequency wrmrzsses; 2"; INVENTORSheldon I. Rambo.

ATTORNEY Patented May 26, 1953 FREQUENCY CONTROL SYSTEM Sheldon I.Rambo, Baltimore, Md ,assignor to VVe'stinghouse ElectricCorporation-East Pittsburgh, Pa., a corporation of PennsylvaniaApplication December 17, 1948, Serial No. 65,959

Claims. 1.

This invention relates to automatic frequency control systems, and moreparticularly to automatic frequency control systems for controlling thefrequencies of an oscillator operating at any predetermined one of .aplurality of frequencies or ,ofapair of discrete oscillators.

It. is frequently desirable and necessary to maintain the frequency ofthe oscillations of an electronic oscillator substantially constant,whether theroscillator is,used for communications or for industrialpurposes. The frequency of the-oscillator can bestabilized by employingaquartz crystal or afrequencycontrolling circuit elementwhich possessesa constant resocillator from a standard .value for; generating controlvoltages, and circuits or devices for retuningthe controlled oscillatorin response to thecontrol voltages.

Systems of'this character have proven satisfactory for controllingoscillators operating at a single predetermined. frequency; However, ifthecontrolled oscillatcris to operate at'more than. asinglepredetermined frequency, the I frequency converting circuits mustbe modified when the oscillatorfrequency is changed; This generallyinvolves-utilization of alternative groups of frequency controlcircuits, and ofmeansfor switching from one group of frequency controlcircuits to another.

Sifnilarly,itis frequently necessary to control the frequencies of apairof oscillators operating on different frequencies.v It has been theusual practice irrsuchcase to control the frequency of eachof theoscillators by means of entirely discrete means.

"It is an object of the present invention to simplify the prior art byproviding an automatic frequency control system of *the'type utilized inaccordance with the'prior .art to control a single frequency oscillator,but in 'whichthe parameters involvedareso selected as to enableeffective frequency control, without any circuit changes whatever, whenthe controlled oscillator is tuned to .seither one of a pair offrequencies.

Anotherobjectof my invention is to provide .an automatic frequencycontrol system which 2 requires no adjustment or modification when thefrequency of the oscillator controlled thereby is changed from onepredetermined frequency to another.

It isstill another object of my invention to provide-a system forcontrolling. the frequencies of either of a pair of oscillatorsoperating on different frequencies by means of asingleautomaticfrequency control circuit.

It is still another object of my invention to provide an automaticfrequency control system which is capable of providing automaticfrequency control for anoscillator operating on either one of twodifferent predetermined frequencies, without requiring a pluralityof-frequency standards or frequency deviation'respon- 'sive circuits.

It is a further object of my invention to provide an automatic frequencycontrol system which, without'any modification or adjustment, is capableof controlling the frequencies of a multiple frequency oscillator, or ofa pair .of'discrete oscillators, and which is, nevertheless, extremelysimple in operation and'design.

Briefly described the invention involves the frequency control of twoalternative frequencies produced by two separate oscillators orbymeansof a singletunable oscillator, the frequencies being either harmonicallyrelated to one another, or both harmonically related to a single commonfrequency, or selected at random.

Either of the two frequencies may,'in accordance with the invention, andwhen they are suitably related, be mixed or heterodyned with a thirdfrequency, equal to the arithmetic mean of the first-mentioned twofrequencies, the same difference frequency being thus derived from thethird frequency in combination with either of the first two frequencies.The-third'frequency may be derived by frequency multiplication from theoutput of a crystal controlled oscillator having a frequency equal tothe difference between the third and eitherof the first two frequencies,so that theoutput of the crystal controlled frequency may be directlycompared with the difference frequency derived from'the mixer in a zerobeat automatic frequency control circuit, which in turn develops an A.F. C. voltage for the first-mentioned oscillator or oscillators formaintaining these very accurately tuned in desired relation to thefrequency vofzthe output of the crystal controlled oscillator.

If the two controlled frequencies are not simply related, or areselected at random, separate frequency controlled oscillators must beprovided 3 for generating one frequency equal to the arithmetic mean ofthe controlled frequencies and a further frequency equal to the productof conversion, or to the difference between the arithmetic mean andeither of the controlled frequencies.

The foregoing, as well as further objects, features, and advantages ofthe present invention, will be made evident by reference to thefollowing detailed description of various specific embodiments of theinvention, especially when taken in conjunction with the accompanyingdrawings, wherein:

Figure 1 illustrates in simplified block diagram, an embodiment of myinvention utilized for controlling two output frequencies of a singleoscillator, these output frequencies being one the second harmonic ofthe other.

Figure 2 illustrates in simplified block diagram a further embodiment ofmy invention similar to the embodiment illustrated in Figure 1, but

wherein one of the control frequencies is a third harmonic of the other;

Figure 3 is a further simplified block diagram of a further embodimentof my invention, having the general character of the embodiment i1-lustrated in Figuresl and 2 of the drawings, but wherein one of thecontrolled frequencies is the nth harmonic of the other controlledfrequency.

Figure 4 is a simplified block diagram of still a further embodiment ofmy invention wherein the two controlled frequencies are not harmonicallyrelated to one another but bear a specified harmonic relation to astandard or controlling frequency;

Figure 5 represents in simplified block diagram still another embodimentof my invention wherein the frequencies of two oscillators arecontrolled in response to the output of a standard frequency source, thetwo controlled frequencies being selected at random except in that thesefrequencies bear each a harmonic relation to a third frequency; and

Figure 6 is a simplified block diagram of a further embodiment of myinvention, wherein the frequencies of two controlled oscillators areselected entirely at random.

Referring now specifically to Figure 1 of the drawings, the controlledoscillator in may be of a type providing output frequencies equal to fand 2f, alternatively, in response to operation of a selective switch S.In such case, the standard frequency source II provides a frequencywhich maybe multiplied by a factor of three in the frequency multiplierI2, the latter then providing an output frequency The output of thefrequency controlled oscillator Il] may beat with the output of thefrequency multiplier I2 in a tuned mixer I3 having an output circuittuned to the frequency i. e., the same frequency as is provided by thestandard frequency source II. It will be evident that an outputfrequency may be provided by the tuned mixer I3 either by conversion ofthe frequency I provided by the frequency controlled oscillator In withthe output frequency provided by the frequency multiplier I2, or bysubtraction of the output of thefrequency multiplier I2 at a frequencyfrom the output of the frequency controlled oscillator I0 at a frequency2!. The output signal of frequency provided by the tuned mixer I3 may becombined with the output of the standard frequency source I I,likewise-equal to I in a zero beat A. F. C. circuit I4, which is ofwell-known character per se. The zero heat A. F. C. circuit I4 will,accordingly, generate a suitable D. C. voltage for controlling thefrequency controlled oscillator II], this voltage being applied to thefrequency controlled oscillator I0 via the leads I5.

It will be evident that an increase of the frequency ,f results in adecrease of the frequency of the output of the tuned mixer I3, whereasan increase of the frequency 2] will result in an increase of suchoutput frequency. Since, how'- ever, the frequency control oscillator I0 requires control voltages of the same polarity to increase itsfrequency whether it is providing an 1 output frequency f or 2f, it isnecessary to apply to the frequency control oscillator I0 an automaticfrequency control voltage of one polarity when the output frequency isf, and of opposite polarity when the output frequency is 2/. To thisend, a polarity reversing mechanism taking the form of a double pole,double throw switch I6 is provided, which is ganged with the frequencyselection switch S, and which serves to apply A. F. C. voltage of onepolarity to the frequency control oscillator I0 when the switch S isthrown to the left to select the frequency ,1, and voltage of oppositepolarity when the switch S is thrown to the right to select a frequency2f.

Referring now to the system illustrated in Figure 2 of the drawings, itwill be evident that the system of Figure 2 and the system of Figure 1are identical, except in that in the system of Figure 2 the controlledfrequencies are equal to f and 3 rather than to f and 2f. Identicalelements in Figures 1 and 2 have, accordingly, been identified byidentical numerals of reference, and where elements are identical exceptin respect to the frequency or frequencies generated or applied theretothe elements have been identified by reference numerals identical withthe reference numerals utilized in Figure 1 except for the addition of asubscript.

In the system of Figure 2, the two frequencies provided being 1 and 3f,the output of the heterodyning frequency applied to the tuned mixer I3afor conversion with the frequencies f and 31, required to equal thearithmetic mean of the frequencies f and 3f, is established at 2/. Thefrequency towhich the output circuit of the tuned mixer 13a is tuned is,accordingly, f, or a value equal to the difference between either 2.2and .f or 3i and 2 The standard frequency source Ha must, accordingly,have a value f in order that it may be compared in the zero beat A. F.C. circuit Ma with the output of the tuned mixer Ilia, and it will beobvious that the input to the tuned mixer l3a at frequency 2f may bederived from the standard frequency source at frequency f by simplemultiplication by a factor of 2.

Referring now specifically to the Figure 3 of the drawings, thefrequency control oscillator, now identified by the reference numeralNib, may be controlled to provide output frequencies of f -or m,alternatively. Accordingly, the system of Figure 3 presents ageneralization of the systems of Figures 1 and 2, the factor n beingequal to 2 in the system of Figure 1, and to 3 in the system of Figure2.

In order that the tuned mixer ltb shall provide an identical frequencywhen the input frequency thereto consists of either the frequency f orthe frequency m, it is essential that a heterodyning frequency equal tothe arithmetic mean of the frequencies f and nf be supplied. Such afrequency may be supplied by the frequency multiplier 12b, themeanarithmetic frequency equaling which is applied to the input circuit ofthe zero beat A. F. C. circuit Hit. The frequency provided by thefrequency multiplier 12b may be r derived from a standard frequencysource ilb which provides a frequency identical with the outputf-reqency of the tuned mixer I32), and accordingly suitable forcomparison therewith in the zero beat A. F. C. circuit Mb, by designingthe frequency multiplier [2b to have a multiplication factor Systemsdisclosed in Figures 1 to 3, inclusive, of the drawings, relate tocontrol of an oscillator to produceaccurately one or another of a pairof frequencies, these frequencies being harmonically related.

In the system of Figure 4, the two frequencies generated by thefrequency controlled oscillator lllc, are no longer harmonically relatedto one another, but they are harmonically related to a common frequencyand are separated by the intervalZf.

Accordingly, the frequency outputs of the frequency controlledoscillator I 00 may 'be generally .described as corresponding with thefrequencies (12:1)7. In order to provide a beat or heterodyningfrequency which, when combined with either of the frequencies (niDf willgenerate the same frequency, the heterodyning frequency must have afrequency value equal to the mean of the values (niDf, or a frequency nThe output of the tuned mixer I 30 must then be tuned to the algebraicdifference of, the frequencies (niDf provided by the frequencycontrolled oscillator 10c, and the frequency nj provided by thefrequency multiplier 12c, i. e., to a frequency f. The frequency ,1 thenis applied to one of the input circuits of the zero beat A. F. C.circuit I40 for comparison therein with a further frequency 1 providedby a standard frequency source He. The heterodyning signal of frequency71. for application to the tuned mixer I30 may be derived from thestandard frequency source i la by multiplication by factor n infrequency multiplier 120.

It will be noted that, in the system of Figure 4, two restrictions existwith respect to the frequencies which may be provided by the frequencycontrolled oscillator iiic. These restrictions are (1) both frequenciesmust be harmonics of a third frequency, and (2) the frequencies mustdiffer by twice this common frequency. In the system of Figure 5, on theother hand, the second limitation has been eliminated. The frequenciesvprovided by two separate frequency controlled oscillators Mid and lliddare selected to be harmonics of a common frequency 1, but either one ofthe frequencies may be constituted of any desired harmonic of thefrequency Accordingly, and for purposes of generalization, the frequencycontrolled oscillator Hid is represented as providing an outputfrequency n while the frequency controlled oscillator !lldd isrepresented as providing an output frequency 111 While, in fact, n and mmay be chosen arbitrarily, it will be realized that one of thequantities m must be greater than the other quantity-n, and selectionhas been accordingly made arbitrarily, for purposes of simplification ofthe exposition of the system, that m shall be greater than n.

The output of the frequency controlled oscillators iiid and Illdd may beselectively applied to an input circuit of the tuned mixer 1311, viaselective switches S, one of which is normally open when the other isclosed, the switches being mechanically ganged to this end. Theheterodyning frequency applied to the tuned mixer [3d must, as in theother embodiments of the invention previously described, provide anoutput frequency equal to the mean of the frequencies n and mf providedby the frequency controlled oscillators Hid and iildd, this meanfrequency being then The output circuit of the tuned mixer l3d must betuned then to the difference between the frequency and either M or my.Assuming, then, that mtis greater than n the output circuit of the tunedmixer I311 must be tuned to the frequency that frequency being thenapplied to one input terminal of the zero beat A. F. C. circuit Md, tothe other terminal of which is applied an identical frequency derivedfrom a standard frequency source having an output frequency Theheterodyning signal provided by the frequency multiplier l2d may bederived from the standard frequency source I id by multiplying thelatter by the factor In each of the systems previously described somelimitation has been placed upon the relation between the frequenciesgenerated by the frequency controlled oscillators or selectivelygenerated by a single frequency oscillator. In the system of Figure 6 onthe other hand, no limitations of any character have been placed on thefrequencies involved, and the frequency controlled oscillator le may,accordingly, be assumed to generate a frequency I while the frequencycontrolled oscillator Hlee may be assumed Such a frequency may besupplied by a standard frequency source lle which may be constituted ofa crystal controlled oscillator. The output circuit of the tuned mixerI3e must, accordingly, be tuned to the frequency difference between l 2and either 1 or F, or must be tuned to the frequency the latterfrequency then being applied to one input circuit of the zero beat A. F.C. circuit Me.

A comparison signal for comparison with the output of the tuned mixerl3e must now be provided by means of a further standard frequency sourcellee, which provides a frequency to the input of the zero beat A. F. C.circuit [46 for comparison with the output signals derivable from thetuned mixer 136.

Since the product frequency provided by the tuned mixer I3e will vary indifferent directions as the output of the frequency controlledoscillator [0e and lflee vary in the same direction, it is necessarythat the automatic frequency control signal provided by the zero beat A.F. C. circuit Me be of different polarity when applied to the separateoscillators I06 and Nice. The A. F. C. voltage derived from the zerobeat A. F. C. circuit 8 He may, accordingly, be applied to the frequencycontrolled oscillator lllee via the leads l5, and to the frequencycontrolled oscillator [0e via the leads [5e which are connected with theleads IE, but which contain at I5j a polarity reversing loop.

The zero beat frequency A. F. C. circuits employed in the variousembodiments of my invention are well known in the art, and require noexposition, per se. Examples of suitable zero beat A. F. C. circuits aredescribed and illustrated in Letters Patent of the United States#2,288,025, issued to A. P. Pomeroy on June 30, 1942, and particularly#2,274,434 issued to C. F. Sheafler on February 24, 1942.

While I have described and illustrated various specific embodiments ofmy invention it will be clear that variations of the arrangementsillustrated may be resorted to without departing from the true spiritand scope of the invention.

I claim as my invention:

1. A high frequency generator comprising, means subject to frequencydeviation for generating oscillations having nominally either of a pairof predetermined frequencies, means for producing further oscillationsof accurately determined frequency combinable with either of saidpredetermined frequencies to produce oscillations nominally of a singlethird frequency, a mixer circuit for combining said further oscillationswith a selected one of said oscillations of predetermined frequencies toproduce said oscillations nominally of said single third frequency,frequency stabilized means for enerating further signals of said thirdfrequency, and means responsive to deviations of the frequency of saidoscillations of said single third frequency from the frequency of saidfurther signals for reducing said deviation by controlling the frequencyof said controlled oscillator.

2. A high frequency generator comprising a controlled oscillator adaptedto generate one of at least two predetermined frequencies, a frequencystabilized oscillator for producing first oscillations of asubstantially constant frequency, means for converting said firstoscillations into further oscillations having a further frequencysubstantially equal to the arithmetic mean of said predeterminedfrequencies, a mixer circuit for heterodyning either of saidpredetermined frequencies with said further oscillations for producingcontrol oscillations of substantially a prechosen frequency, andfrequency control circuit means responsive to frequency deviations ofsaid control oscillations relative to the frequency of said firstoscillations for adjusting the frequency of said controlled oscillatorsuch that said deviations are thereby reduced.

3. A high frequency generator comprising a frequency controlledoscillator adapted to operate on one of at least two predeterminedfrequencies, a standard oscillator for producing oscillations ofconstant frequency, means for converting said oscillations into furtheroscillations having a second frequency equal to the arithmetic mean ofsaid two predetermined frequencies, means for heterodyning either one ofsaid predetermined frequencies with said further oscillations to produceoscillations nominally of said constant frequency and subject to afrequency deviation :Af, a zero beat comparison circuit for comparingsaid last-named oscillations with said constant frequency oscillationsto generate a frequency control voltage substantially proportional to1A], and means responsive to said frequency control voltage foradjusting the frequency of said controlled oscillator to reduce saiddeviations in 4. A high frequency generator comprising a controlledoscillator for producing oscillations having either of two frequencies(nl) fill and (n+1)J-:Af, where (n-l)f and (n-l-Df are predeterminedfrequencies, with n being any integer, and :Af represents a variablefrequency deviation, frequency stabilized means for producingoscillations of frequencies f and M, a mixer cir cuit for selectivelycombining said oscillations of frequency 11, with said oscillations offrequency (n-DjiAf or (n+l)fiAf to produce resulting oscillations offrequency fin), and automatic frequency control circuits responsivejointly to said oscillations of frequency ,fiAf and said oscillations offrequency for adjusting said controlled oscillator to reduce saiddeviations iAf.

5. In combination, a source of frequency controllable oscillations ofeither of frequencies J and F, means for generating frequency stabilizedoscillations of frequency means for combining either one of saidfrequencies f and F with said frequency stabilized oscillations offrequency to produce comparison oscillations of frequency means forgenerating frequency stabilized oscillations of frequencies and a zerobeat automatic frequency control circuit for comparing the frequency ofsaid lastnamed frequency stabilized oscillations with the frequency ofsaid comparison oscillations and to generate an automatic frequencycontrol voltage for said source of frequency controllable oscillationsin response to frequency differences between said last-namedfrequencies.

6. In combination, frequency controllable oscillators arranged togenerate either of frequencies 11. or my, where m n, and m and n areintegers, a frequency stabilized source of oscillations of frequencymeans for multiplying the frequency of said oscillations by a factor togenerate further oscillations having a frequency nf-l-mf 2 means forcombining either one of said frequencies mf or nf with said frequency toproduce comparison oscillations at a frequency means for combiningeither one of said frequencies ,f or F with said frequency 2 to producecomparison oscillations at a frequency f-F means for generating furtherfrequency stabilized oscillations of frequency a zero beat automaticfrequency control circuit for comparing the frequency of said furtherfrequency stabilized oscillations with the frequency of said comparisonoscillations and to generate an automatic frequency control voltage forsaid frequency controllable oscillators in response to frequencydifferences therebetween.

8. In combination, a frequency controllable oscillator arranged togenerate either of frequencies or m, where n is any integer, a frequencystabilized source of oscillations of frequency means for multiplying thefrequency of said oscillations by a factor n+1 i i to generate furtheroscillations having a frequency means for combining either one of saidfrequencies f or ii with said frequency to produce comparisonoscillations at a frequency a zero beat automatic frequency controlcircuit for comparing the frequency of said oscillations with thefrequency of said comparison oscillations and to generate an automaticfrequency control voltage for said frequency controllable oscillator inresponse to frequency differences therebetween.

9. A high-frequency generator comprising, a source of high frequencyoscillations adapted to generate either first oscillations or secondoscillations from a plurality of predetermined frequencies, a frequencystabilized oscillator for pro- 1 l a ducing further oscillations of asubstantially constant frequency, said constant frequency being thearithmetic mean of said first and second oscillations, mixer means forcombining said first oscillations and said further oscillations toproduce resulting oscillations of a third frequency, a source ofstabilized prechosen frequency oscillations, said last-namedoscillations having substantially the same frequency as said resultingoscillations, and frequency control means responsive to deviations ofsaid resulting oscillations relative to said prechosen frequencyoscillations.

10. In combination with a high-frequency generator, means for generatingfirst oscillations having either one of a pair of predeterminedfrequencies, means for generating second oscillations of a stabilizedprechosen frequency, a mixer circuit for combining said secondoscillations with said first oscillations to produce third os--frequency control means responsive to deviationsof said thirdoscillations relative to said pre-' determined frequency oscillationsfor reducing such deviation by controlling the frequency of said firstoscillations.

SHELDON I. RAMBO.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,450,966 Affel Apr. 10, 1923 2,406,125 Ziegler et al Aug. 20,1946

